There are many subterranean petroleum-containing formations from which petroleum cannot be recovered because the petroleum viscosity is so high that it will not flow or cannot be pumped to the surface of the earth without first applying a treatment which accomplishes a reduction in the petroleum viscosity. The most extreme example of viscous petroleum-containing formations are the so-called tar sand or bituminous sand deposits. The largest and most famous deposit of tar sand is the Athabasca tar sand deposit located in the northeastern part of the province of Alberta, Canada. Although this deposit contains in excess of 700 billion barrels of petroleum, essentially no recovery of petroleum has been affected by commercial means from these deposits because of the very high viscosity of the oil contained therein. Other viscous oil formations are found in the United States and in various other countries throughout the world.
Thermal recovery techniques have been used successfully for recovering viscous petroleum from subterranean formations in many applications, although they have been unsuccessful on a commercial basis in other deposits for a variety of reasons. The most successful thermal recovery technique involves introducing steam into the formation to raise the temperature of the viscous petroleum, thereby decreasing its viscosity sufficiently that it will flow or can be displaced to a well, which may be the same well as was used for steam injection or a spaced-apart production well, from which it can be pumped to the surface of the earth. Although most viscous oil formations can be stimulated to produce some oil by steam injection, the cost effectiveness is such that steam flooding can be applied to viscous oil formations on a profitable basis in only a limited number of instances. The principal cost factor in steam flooding operations is the cost of fuel required to generate the steam for injecting into the formation. Obviously, the amount of oil recovered per unit of fuel required to generate steam used in the recovery of oil is a critical factor, and is the principal reason that many viscous oil formations cannot be successfully exploited by steam stimulated recovery.
Various additives have been proposed in the prior art for improving the effectiveness of steam flooding oil recovery processes. Various solvents have been injected or mixed with steam, and generally result in some improvement in the oil recovery, although it has often been found that the amount of additional oil recovered is not sufficient to justify the cost of the solvents introduced into the formation in combination with steam. The use of liquid, gaseous, and combinations of liquid and gaseous hydrocarbon solvents with steam are disclosed in many prior art references.